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There are several issues to consider here:
1) Weight. Can the roof handle the additional weight or should the old shingles be removed? Copper roofing is fairly low weight in comparison to most other roofing materials but, still, what the building can handle should be considered. If there are current signs of a weight problem (sagging decking, buckled trusses), the old shingles probably need to come off.
2) Substrate. Is the roof decking in good condition so that the new roof can be fastened securely? If there have been roof leaks or other problems, any rotted or dertiorated decking needs to be addressed.
3) Building Codes. Many building codes restrict to no more than two layers of roofing. If there are two layers now, you will need special approval for a third. Once you explain the low weight of copper to the building inspector, though, that might still be possible.
4) Ventilation. Adding new underlayment and shingles will make the roof even tighter. You need to make sure that proper attic ventilation is in place to avoid a build up of moisture or heat in the attic. (This is just a construction principle and is not specific to metal roofing.)
5) Design and current shingle condition. Some copper shingles are more forgiving than others for installation over uneven old shingles. I would check with the manufacturer of the copper for their opinion on this. And, in any event, a new underlayment should be installed over the old shingles. That can help to smooth things out a bit.
10/14/2002
Or copy this URL: https://www.metalroofing.com/spirit/comment/1261/find/Isaiah Industries, Inc.
10/14/2002
Or copy this URL: https://www.metalroofing.com/spirit/comment/1262/find/